The environmental costs of fossil fuels and the political instability of oil-producing regions have intensified efforts to develop alternative energy sources that are environmentally clean, more efficient, and more reliable. Wind-driven power generation systems are of particular interest in part because they are currently the one type of renewable power source that is closest to being economically competitive with traditional fossil fueled sources. Wind power may be converted to electrical power using a rotor assembly, either horizontally or vertically oriented to the flow of the ambient wind. The rotor blades of the rotor assembly convert the energy of the moving air into rotational motion on a drive shaft of the rotor assembly. An electrical generator coupled to the drive shaft then converts the rotational motion into electrical power.
Conventional wind-driven power generation systems suffer from many challenges. The term conventional is meant generally herein to describe a system that comprises a monopole tower with a single multi-bladed rotor spinning about an axis that is horizontal to the ambient flow of wind and is located at or near the top of the tower, i.e., a horizontal wind axis turbine or “HAWT” system. In general, wind power generators operate only when the wind blows, only within a certain range of wind velocities, and at a maximum power output level for an even smaller range of wind velocities. As a result, wind power generation has traditionally been expensive to produce and not reliably available. In response, conventional wind turbine manufacturers' assemblies have evolved towards very large rotor assemblies—with rotor diameters often equal to or greater than 90 meters—and very tall towers in order to gain economies of scale and to reach higher velocity and steadier winds that occur at higher altitudes.
However, increases in the sizes of conventional rotors have led to a number of additional problems. Large rotors are much more difficult to manufacture because the size of each blade reduces the ability for mass production and because the forces on the blade require special and expensive materials. Delivery of large rotors to the generation site is also a severe problem which often requires specialized trucking systems, assistance clearing crowded roadways, and wider/longer access ways near the wind farm which are often not feasible given the wind farm's remote location and position on hillsides. Maintenance is a challenge given the inability to quickly and easily access damaged components and the inability to deliver replacement components quickly. In addition, large rotors create greater torque and balance problems on the nacelle hub which therefore requires stronger gear box assemblies that often require exotic alloy compositions.
Another well-known problem of traditional turbines is the footprint such a technology requires. Wind rotors require smooth wind for maximum conversion efficiency. Turbulence from adjacent rotors forces towers to be spread across great distances to allow winds to recover optimal wind properties. The larger the rotors, the greater the turbulence, and the fewer the number of towers that can be placed on a given wind farm acreage. Additionally, operational efficiency is diminished due to the inability of such large rotors to effectively accommodate heterogeneous wind conditions at different altitudes across the rotor's face. In other words, it is difficult for a single large rotor to handle winds that come from different directions and/or at different speeds within the diameter of the single rotor.
Alternatively, an augmented wind power generation system uses a funneling apparatus, for example a fully or partially shrouded rotor, to increase the velocity of the ambient wind—based on the physics of the “Bernoulli Effect”—across the rotor blades. Because the electrical energy that is generated from a wind turbine is a cubic function of the speed of the wind, an augmented wind generation system holds the promise of producing an equivalent amount of electrical power from a much smaller rotor assembly. Funneling apparatuses may be vertically stacked into a tower with one or more rotor assemblies located within each apparatus. There are numerous types of wind amplification devices, but some are described in U.S. Pat. No. 4,156,579 (Weisbrich), U.S. Pat. No. 4,288,199 (Weisbrich), U.S. Pat. No. 4,332,518 (Weisbrich), U.S. Pat. No. 4,540,333 (Weisbrich), U.S. Pat. No. 5,520,505 (Weisbrich), and U.S. Pat. No. 7,679,207 (Cory). All six of the above patents are hereby incorporated by reference as if fully set forth herein.
Specific benefits of the exemplary embodiments are expanded upon later, but in general the use of smaller rotors to produce an equivalent amount of energy to larger rotors produces numerous advantages for augmented systems over conventional wind turbine systems. First, smaller turbines are easier to mass produce and easier to transport to the generation site. Second, smaller rotor diameters require a smaller diameter of wind flow to operate which reduces the inefficiencies of having heterogeneous wind conditions at different altitudes. Third, “cut in” speeds, the speed at which a turbine begins to generate electricity, are lower because the smaller blades are usually lighter and more able to operate in lower wind conditions. Fourth, the footprint of towers with smaller rotors is significantly improved because the size of the turbulence field is smaller, and towers can be placed much closer together. Fifth, the torque impact on the hub gears is significantly lessened thereby reducing the need for heavier engineering and exotic materials. And sixth, the smaller rotors spinning at higher speeds located next to a partial or full shroud provide the visual signals needed for birds to avoid the path of the rotor blades thereby reducing accidental bird kill often found with traditional tower systems.
However, not all augmented systems offer equal benefits. Some augmented systems are still not optimized to fully maximize wind amplification efficiently or to do so in an economically ideal manner. An illustrative example would include a recent system described in U.S. Pat. No. 7,679,209. This tower utilizes a single, cylindrical core to create marginal wind amplification and a series of cylindrical rotors on only two shafts adjacent to the one core to convert wind energy into electrical energy.
This configuration is not optimal for energy conversion in an economically attractive way for a variety of reasons. First, unlike a toroidal shaped tower the single cylindrical tower only deflects wind in one direction (laterally) thereby providing only marginal increases in wind speed. In other words, a much greater amount of wind is captured and funneled in a more efficient way in a toroidal tower leading to higher volumes of faster wind streams. Second, the cylindrical rotor assembly is only a small portion of the overall tower silhouette leading to unnecessary tower costs and additional loss of potential wind energy on a given wind farm. Third, the single open cylinder exposes key components to the elements leading to unnecessary and expensive maintenance costs. Fourth, the rotors are connected to only two rotating shafts meaning that there are relatively few (two) generators for the whole apparatus which thereby restricts the tower's generating capacity. Fifth, because there are only two vertical shafts for the entire tower it is unable to have rotors automatically face the direction of the wind flow at different altitudes which again reduces the tower's applicability in larger sizes as well as its economic efficiency. Although an interesting idea for enhancing ambient wind speeds, the systems described in U.S. Pat. No. 7,679,209 and others like it fail to effectively compete with conventional large scale rotor systems or with the embodiments described herein.
One system that has withstood the test of time because of the potential of its unique attributes is the toroidal shaped wind tower introduced in U.S. Pat. No. 4,156,579 (Weisbrich) and expanded upon in U.S. Pat. No. 5,520,505 (Weisbrich), and U.S. Pat. No. 7,679,207 (Cory). This configuration uses a series of vertically stackable, partially shrouded tower modules to direct wind over a pair rotor systems located within the hollows of each module.
There are numerous benefits of this configuration. First, substantial research of the toroidal shape has demonstrated the efficiency of this configuration due to a substantial increase in wind speed especially nearest the sides of the core tower. The unique shape allows for a large volume of wind to be funneled from three sides (top, bottom, and laterally), not just one, towards the rotors in the hollows of the tower. Second, the rotor pairs operate independently from rotors on other levels which allow each rotor pair to face directly into the wind at its particular altitude. Third, because each pair is independent from other pairs, the tower can produce a portion of its overall capacity when wind conditions and maintenance activities warrant. For example, if the wind is moving sufficiently fast to generate power at the level of the higher modules but not at the lower modules, the higher module rotors can still operate and produce electricity thereby increasing the tower's overall “capacity factor”—i.e., actual energy output per year compared to the potential maximum. Partial production is also a significant advantage to reduce maintenance costs because the tower can still produce some power while a subset of rotors is being fixed. A conventional wind rotor needs a sufficient average wind flow across its entire, large diameter area to generate any power at all leading to an all or nothing output of electricity. Partial production is an advantage for the toroidal tower configuration over traditional towers and over other amplified systems such as those found in U.S. Pat. No. 7,679,209. Fourth, the toroidal tower configuration is also scalable allowing the generation of power in the multi-megawatt scale. And fifth, the toroidal tower allows multiple rotors to be accessed and maintained on a single tower reducing maintenance costs for large scale towers. These benefits are in addition to the general advantages of augmented systems described above such as smaller footprint, lower cut in speeds, reduced gear box requirements, lower cost from mass production, and reduced bird kill.
Although the toroidal configuration has some clear advantages even over other augmented systems, its success in being effectively commercialized has been limited to date. A key factor constraining the adoption of the technology thus far has been the inability to manufacture a rotor and generator system to fully capitalize on the unique horizontal wind shear flow pattern purposefully created by the toroidal tower. As illustrated in FIG. 1, a horizontal wind shear exists due to the Bernoulli Effect wherein the speed of the wind nearest the shell of the toroidal tower is traveling faster than the wind located further away from the tower. In other words, the further from the tower the wind flow is located, the lower the amplification effect and therefore the lower the wind speed. To date, engineers have relied on using traditional HAWT rotors on toroidal towers, but these rotors are severely hampered by the strong horizontal wind shear environment. Specifically, the blades of the HAWT rotor travel perpendicularly through the horizontal wind shear such that the blade nearest the tower is receiving a much greater force of wind than those blades further from the tower. In addition, the outer tip of an individual blade is impacted much more severely by the wind shear than the portion of the blade nearer to the center axis. Both of these factors create differential torque on the individual blades and the overall rotor causing significant decreases in efficiency and increases in rotor malfunctions/failures.
The embodiments described herein provide a unique combination of adaptations to a traditional Vertical Axis Wind Turbine (“VAWT”) and generator configuration that specifically accommodate and optimize the benefits of a multilevel toroidal tower towards the goal of cost effective, large scale power production.
VAWT systems are a type of wind turbine where the main rotor shaft is vertical instead of horizontal to the ambient flow of the wind. For illustration purposes, a traditional wind mill with three propeller blades connected to a generator behind the blades is a HAWT assembly while an anemometer that uses cupped paddles to measure wind speed is an example of a Savonius style VAWT assembly. Advantages of VAWT rotors over HAWT rotors tend to include omni-directional operation (can simultaneously accept wind from any direction), low noise, and excellent durability even in turbulent wind conditions. Traditional disadvantages of VAWT have included lower conversion efficiencies and a pulsatory torque that is produced during each revolution. Later approaches solved the torque issue by using a helical twist of the rotor blades. In addition, because VAWT rotors have traditionally been more difficult to mount on a monopole tower they are often installed nearer to a base or the ground which typically results in access to lower speeds and more turbulent winds.
A subset of the VAWT family of wind generators is the Savonius system. A Savonius VAWT system is a “drag” type of device with two (or more) scooped blades like those used in anemometers. Savonius VAWT wind rotors spin because there is a differential in pressure between the convex and concave side of the cupped blade. In a horizontal wind shear environment—such as that which is created in a toroidal wind tower—a VAWT rotor would benefit by the differential speeds of the funneled wind in that the faster winds near the tower would be hitting the concave or cupped portion of the blade while the slower winds away from the tower would provide relatively less resistance against the convex portion of the blade as it spun around its central axis. As a result, the VAWT rotor should be more efficient than normal in a horizontal wind shear environment and should, more importantly, avoid the challenges faced by a HAWT rotor which cuts perpendicularly through the wind shear turbulence.
In addition, a complementary component that should improve the efficiency, output, and cost of the proposed VAWT rotor is the recent commercialization of permanent magnet synchronous (“PMS”) generators now being manufactured by multiple companies. These generators allow for a direct connection between a drive shaft and the generator such that complicated and costly gear boxes can be eliminated. In addition, the smaller sizes of PMS generators are particularly advantageous to the smaller rotors and higher rotation speeds of an augmented wind power generation system. Taken in concert with the potential benefits of using a continuous variable transmission (“CVT”) drive, as described in U.S. Pat. No. 7,679,207 (Cory), the new PMS generators should help to significantly improve and extend the power curve of a rotor assembly in an augmented wind power generation tower although such benefits are not necessary for the benefits of the current patent to manifest.
Although a traditional VAWT rotor should be uniquely positioned to take advantage of a horizontal wind shear environment, the rotor blades and the integration of the rotor system with its associated generator and components must be significantly reconfigured to economically accommodate a large scale toroidal shaped wind tower and to enhance conversion efficiencies that have limited previous VAWT rotors. The focus of the embodiments disclosed herein is to present such a improvement.